The present application is based upon and claims priority from Japanese Patent Application No. Hei. 3-336409 filed Oct. 22, 1991, the disclosure of which is incorporated by reference herein.
The present invention relates generally to a zoom lens system that is suitable for use with compact cameras which have a smaller constraint on back focus than single-lens reflex cameras. More specifically, the present invention relates to a zoom lens system that features a high zoom ratio of 2.5 and more.
Various types of zoom lens systems have heretofore been known for use with compact cameras. Zoom lenses consisting of three lens groups or more with a zoom ratio exceeding 2 are categorized as follows:
(i) Four-group zoom lens system comprising four lens groups (positive, negative, positive and negative groups), with a stop diaphragm being provided between the second and third groups, all lens groups being moved independently of each other towards the object (this type includes a system where some of the four lens groups are moved in unison). Examples of such zoom lens systems are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-43115, No. SHO 63-159818 and No. SHO 63-157120.
(ii) Three-group zoom lens system comprising three lens groups (positive, positive and negative groups), with a stop diaphragm being provided in the second group, all lens groups being moved independently of each other towards the object. Examples of this zoom lens system are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-153511 and No. SHO 63-161423.
(iii) Three-group zoom lens system comprising three lens groups (positive, positive and negative groups), with a stop diaphragm being provided between the second and third groups, all lens groups being moved towards the object (see, for example, commonly assigned U.S. Pat. No. 4,978,204)
(iv) Practically four-group zoom lens system that has the same composition as the system (iii) except that the second group is divided into a front and a rear group that are movable independently of each other (see Example 3 in the specification of U.S. Pat. No. 4,978,204).
The above-described conventional zoom lens systems have their own problems. In the system (i), all of the four lens groups have to be moved independently of each other, so a large number of cams must be used; however, it is mechanistically difficult to accommodate those cams in the small space available for lenses for use with a compact camera.
The systems (i) and (ii) require that a shutter block also serving as a stop diaphragm be disposed either between the second and third lens groups (which are subject to substantial deterioration in performance due to manufacturing errors) or within the second group. Under the circumstances, it is difficult to insure high mechanical precision for the central axis of the lens groups and, at the same time, it is difficult to assure consistent optical performance since the imaging performance will be deteriorated greatly in the presence of slight errors.
In addition, the last negative lens group in the conventional three- or four-group zoom lens system has a greater power than the other lens groups. Further, it comprises, in order from the object side, a positive, a negative and a negative element or a positive, negative and a positive element, to form a three-group-three-element or a two-group-three-element composition. Thus, a large number of lens elements must be used and each lens element has a great power, causing the problem that the overall lens system will suffer from a great deterioration in performance due to manufacturing or assembly errors or from a large focus shift due to temperature or humidity changes. These phenomena of performance deterioration and focus shift are prone to become extensive as the overall lens length decreases.
Another feature of the prior art zoom lens systems is that in order to correct aberrations, an optical glass having a small Abbe number is used in the first lens element of the last lens group whereas two or more negative lens elements of a large power are provided in the rear part; however, this has increased the number of constituent lens components.
The present invention has been accomplished in the light of the aforementioned problems of the prior art and has as an object providing a zoom lens system that features a high zoom ratio of 2.5 or more, that is short in the overall lens length, that is composed of a smaller number of lens elements, that will experience less deterioration in performance due to manufacturing or assembly errors or less focus shift due to temperature or humidity changes, and which hence is suitable for use with a compact camera.